Accurate Determination of Carbon-Foil Surface Densities

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Abstract
We have developed a method of accurate measurement of the surface densities of carbon foils prepared by vacuum evaporation, and have studied some of the properties of such foils in the range 0–70 μg/cm2. The method consists of burning a sample foil in a stream of pure oxygen, trapping the carbon dioxide thus formed, and measuring the pressure of the trapped gas in a known volume. We find that T, the optical transmittance at λ546 nm of a foil on a microscope slide substrate, indicates the foil's carbon content to within about 10% over the range 5–60 μg/cm2 and is given approximately by the relationship T=0.687 [exp(0.0911 σ) + 0.0273 exp(−0.0911 σ) −0.330 cos2π (0.089+0.055 σ)]−1, where σ=relative surface density with 1 μg/cm2 taken as unity. Such foils behave nearly as simple conducting layers having a complex refractive index 2.75+0.720i at λ546 nm. Our unbacked carbon foils have a noncarbon component with a surface density of 1.6±0.5 μg/cm2, approximately independent of foil thickness. This component may cause significant errors in other kinds of measurements of the thicknesses of foils prepared by evaporation.

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